Photographic silver halide emulsion



I lrn ll -o UNITED STATES PATENT OFFICE PHOTOGRAPHIC SILVER HALIDE EMULSION No Drawing. Application October 30, 1941, Se-

rial No. 417,165. In Great Britain February 14,

9 Claims.

This invention relates to photographic silver halide emulsions in which polyvinyl alcohol is employed as the protective colloid for the silver halide therein. This invention also relates to photographic silver halide emulsions in which a polyvinyl alcohol gelled by means of a substituted hydroxy aromatic compound such as orcinol, gallic acid, 2-4 dihydroxy benzoic acid or 4-chlor resorcinol, or by a substituted monohydric naphthol such as 5-amino-l-naphthol hydrochloride, is employed as the protective colloid. This is a continuation-ln-part application of my U. S. application Serial No. 318,559, filed February 12, 1940, now Patent No. 2,286,215.

Gelatin has ordinarily been employed as the protective colloid for the light-sensitive silver halides in photographic emulsions due to its ability to hold these salts in suspension, its good water permeability, its insolubility in water and in photographic developing solutions at ordinary temperatures and its ability to form a thin coating of the light-sensitive silver halide emulsion upon the film base.

Various other materials have been suggested for use as the protective colloids in photographic emulsions but in most instances the suggested materials have evidenced inferiority in one or more of the properties enumerated, compared with gelatin. For instance, cellulose esters, such as cellulose nitrate, have been suggested as carriers for silver salts in photographic emulsions. The emulsion layer formed, however, when dry, is none too permeable to developing solutions and, therefore, cellulose nitr ate has not been successful with the conventional developing and fixing baths. Another disadvantage of cellulose nitrate is its insolubility in solvent mixtures containing appreciable amounts of water. This prevents the use of the water-soluble salts which are now ordinarily employed in forming light-sensitive photographic emulsions.

Various other colloidal materials have been mentioned in this connection. However, most of those that have been suggested heretofore are deficient for some reason or other, such as lack of good water permeability, inability to hold silver halide in suspension or inability to form a hard emulsion layer when coated upon a support, such as photographic film base or paper, and dried.

Gelatin as a protective colloid. for photographic silver halide emulsions leaves much to be desired. Gelatin is susceptible to depredations by organisms, both animal and vegetable. For instance, insects attack gelatin, particularly in the tropics. Gelatin is susceptible to the action of molds and of bacteria, particularly under conditions of high humidity and a compatible temperature. Gelatin, because of its natural source often varies in properties from time to time. Also, to obtain gelatin in a highly refined condition (as needed for photographic purposes) requires considerable care and it can be procured in limited amounts only from animal products by a slow and expensive process.

One object of my invention is to provide carriers or protective colloids for light-sensitive silver halides which can be employed instead of gelatin for various photographic silver halide emulsions. Another object of my invention is to provide carriers or protective colloids for lightsensitive silver halides in photographic emulsions which possess many of the advantages, but lack some of the disadvantages evidenced by gelatin. Other objects will appear herein.

I have found that polyvinyl alcohol is eminently suitable as a carrier for the silver halide in photographic emulsions, the physical properties of the material being easily controlled in production so as to get a material of constant properties. Polyvinyl alcohol has the ability to hold silver halide in suspension, is easily coated out upon a support such as paper, readily sets and dries to a hard, scratch-resistant emulsion layer which is susceptible to processing by means of conventional developing and fixing baths, and which may be washed thoroughly with water after processing. I have found that a polyvinyl alcohol emulsion layer swells, but does not dissolve when developed, fixed, and washed in the usual way. After drying, it exhibits good physical properties. I have found that photographic silver halide emulsions having polyvinyl alcohol as the protective colloid in which an amount of a substituted hydroxy aromatic compound such as orcinol or gallic acid or a substituted monohydric naphthol has been incorporated therewith to cause gelling of the polyvinyl alcohol, have good photographic properties. The treatment of an aqueous solution of polyvinyl alcohol with a substituted hydroxy aromatic compound or a substituted monohydric naphthol to convert the polyvinyl alcohol to a rigid gel, which is thermal-reversible, is described and claimed in U. S. Patent No. 2',249,536 of McDowell and Kenyon. This phenomena is distinct from the hardening action evidenced by some materials on polyvinyl alcohol.

A photographic silver halide emulsion in accordance with my invention may, for instance,

5-10% concentration by treating the mass with.

a substituted hydroxy aromatic compound or a substituted monohydric alcohol at a slightly ele vated temperature (preferably 40-90 C.) in an amount which, when cooled, forms a rigid thermal-reversible gel. After the cooling has taken place and the emulsion gel has been formed, it may be washed or otherwise treated as desired. The phenomena described is valuable in that it makes possible the coating of the photographic emulsion upon a support while the mass is liquid at the elevated temperature and then gel-setting of the emulsion. The emulsion can then be dried and forms a photo-sensitive layer on the support providing, of course, that this operation was carried out in the absence of light. The gelling agent should only be added after the formation of the emulsion to satisfactorily form a gelled product. It is preferable in incorporating a gelling agent in a photographic emulsion that the amount be restricted to that which will not crystallize out from the emulsion upon drying unless, of course, the final product is employed in some operation where the presence of those crystals is not objectionable.

The term polyvinyl alcohol used herein includes any of the water-soluble hydrolyzed polyvinyl esters having a polyvinyl ester content of approximately 5% or lessthe remainder being polyvinyl alcohol-as well as that compound which is made up entirely of polyvinyl alcohol without any ester groups. These compounds are particularly adapted to the preparation of unwashed emulsions and when coated on a suitable support such as paper in the form of a photosensitive silver halide emulsion will record images accurately and sharply, forming permanent prints when prepared in the ordinary manner.

Polyvinyl alcohols are eminently suitable for the preparation of washed photographic emulsions in accordance with my invention. These emulsions are particularly suitable for coating onto a cellulose ester support and when so coatcd in the form of a photosensitive silver halide emulsion will, upon proper exposure, developing, and printing give a sharp image.

The photographic emulsions of my invention may be prepared by a process involving first dispersing silver halides in polyvinyl alcohol such as by running aqueous solutions of silver nitrate and alkali metal halide into an aqueous solution of polyvinyl alcohol, preferably after adding a gelling amount of a substituted hydroxy aromatic compound such as orcinol or gallic acid or a substituted monohydric naphthol and warm ing to cause better dispersion and increase the speed as is well known in the art, following by cooling and, if desired, sensitizing with a small amount of a sensitizing dye, the emulsion is ready to coat out onto a support such as paper The emulsion is coated out in a warm condition so as to render it flowable, After the emulsion layer is dry, such as by a current of warm air. it may be exposed, processed, washed, and dried in the usual manner and prints of good appearance result. Where the gelling agent is used, the emulsion, after coating out onto a support, can be set by cooling.

It is preferable to incorporate the gelling agent in the emulsion composition after the formation of the silver halide because otherwise, there may sometimes be a tendency toward fog.

ill

The gelling agent may be added after the addition of the polyvinyl alcohol which is mixed with the silver halide. In the case of the washed emulsions, it is often desirable to add some of the gelling agent thereto after the washing operation to replace any gelling agent which was lost in that operation.

As described in the McDowell and Kenyon Patent No. 2,249,538, certain of the substituted hydroxy aromatic compounds and the substituted monohydric naphthols convert polyvinyl alcoho] to a thermal-reversible gel. This gelling agent is incorporated in an aqueous solution of the polyvinyl alcohol at a temperature above the gelling point of the polyvinyl alcohol. Then upon cooling, the aqueous solution of polyvinyl alcohol forms a gel. Ordinarily, the photographic emulsions which employ polyvinyl alcohol are aqueous solutions or suspensions containing approximately 3-5% of the polyvinyl alcohol. Aqueous polyvinyl alcohol which has been gelled is a material somewhat similar to gelatin in many of its physical properties in that it is non-tacky, easily cut, easily handled, and can be shredded or noodled, In this noodled form it is readily susceptible to washing and working up as is common with gelatin photosensitive emulsion. The gelling agents listed herein are therefore distinguished from hardeneding agents of polyvinyl alcohol as the latter cause the polyvinyl alcohol to become hard and relatively impermeable and to assume a form, which does not readily permit working up as does polyvinyl alcohol which has been converted into a permeable thermal-reversible product.

The amount of substituted hydroxy aromatic compound or substituted monohydric naphthol employed in photographic emulsions in accordance with my invention need be only a gelling amount. For instance, the usual emulsions using polyvinyl alcohol as the protective colloid as finely prepared, contain about 3-5% of polyvinyl alcohol. For 50 grams of such an emu sion, the amount of gelling agent usually employed is approximately /5 to of a gram varying with the different gelling agents. For instance, with gallic acid, of a gram is suflicient to give a product of satisfactory properties. Using orcinol, it is desirable to employ of a gram to obtain the desired properties. With other gelling agents, the amount employed will vary depending on the gelling agent used and the characteristics desired in the gelled polyvinyl alcohol. In preparing a washed emulsion, some of the gelling agent is usually washed out and, therefore, it is desirable to employ a greater amount of the gelling agent than where the washing operation is not present. Also, after the washing operation, the addition of a small amount of gelling agent is desirable. It is desirable, however, that the amount of gelling agent employed be near the amount necessary to accomplish the desired purpose so as to eliminate any danger of formation of crystals of the gelling agent when the emulsion is dried and also to prevent too great a rigidity in the product which may result in excessive hardness or a melting point higher than necessary.

My invention is concerned with the preparation of photographic silver halide emulsions adapted to be coated upon paper or upon transparent base such as cellulose derivative film base or glass plates to form what is commonly known in the art as photographic paper, photographic film, or photographic prints.

The photographic silver halide emulsions of my invention may be sensitized in the same manner as are silver halide emulsions generally. This is ordinarily accomplished with any emulsion by thoroughly incorporating a small amount of a sensitized dye after the emulsion is prepared.

The photographic silver halide emulsions of my invention may be sensitized in the same manner as are silver halide emulsions generally. This is ordinarily accomplished with my emulsions by thoroughly incorporating a small amount of a sensitizing dye after the emulsion is prepared. If staining of the paper on which the emulsion is coated is objectionable, an amount of sensitizing dye, which is not great enough to cause staining, may be employed. If a more sensitive emulsion is desired and staining is not objectionable, or a support which will not absorb the dye is used, a larger proportion of sensitizing dye may be incorporated. The emulsions, containing the smaller amounts of sensitizing dye, are ordinarily adapted for making contact prints or enlargements by projection.

Th'e sensitizers, which have been previously employed to optically sensitize silver halide emulsions, may be incorporated in the emulsions of my invention to increase their sensitivity. Examples of sensitizers, which may be employed in the emulsions described herein, are the cyanines and the sensitizing dyes disclosed in the following patents:

U. S. No. 2,078,233, Brooker, April 2'7, 1937; U. S. No. 2,166,736, White et al., July 18, 1939; U. S. No. 2,186,608, Keyes, January 9, 1940; U. S. No. 2,165,339, Brooker, July 11, 1939.

The following examples illustrate my invention:

Example I 120 cc. of a 10% solution of polyvinyl a1 Cohol was diluted with 140 cc. of water. Into this the following solutions were run with vigorous stirring, the precipitation time being five minutes. Solution A: 40 cc. of a 50% solution of silver nitrate, Solution B: 12.8 grams of potassium bromide, 3 grams of potassium chloride, 0.5 gram of potassium iodide and 44 cc. of water. 80 cc of a 10% solution of polyvinyl alcohol was then add d and the emulsion was stirred for 20 minutes at 50 C. 120 cc. of a 10% solution of polyvinyl alcohol was added and the emulsion was cooled. 50 grams of this emulsion was heated to 40 C. and of a gram of gallic acid was added. One milligram of 2,3-diethyl-4'-methyl-oxathiazolo-carbocyanine dissolved in a little methyl alcohol was added to sensitize the emulsion. The emulsion was coated on a glass plate and found to gel in 2-3 minutes at room temperature and more rapidly when chilled. The plate was exposed wet and processed. It was found to develop and fix readily.

A plate coated as before was washed by placing in cold water for 30 minutes and was then dried. On exposing and processing, the plate was found to develop and fix readily giving a clear, clean result. The emulsion was also coated on paper. It was dried by subjecting to a current of warm air. Upon exposing and processing, it was found to give a clear, clean result.

Example. II

120 cc. of a solution of polyvinyl alcohol was diluted with 140 cc. of water. Into this the following solutions were run with vigorous stirring, the precipitation time being five minutes.

Solution A: 40 cc. of a 50% solution of silver nitrate, Solution B: 12.8 grams of potassium bromide, 3 grams of potassium chloride, 0.5 gram of potassium iodide and 44 cc. of water. cc. of a 10% solution of polyvinyl alcohol was then added and the emulsion was stirred for 20 minutes at 50 C. cc. of a 10% solution of polyvinyl alcohol was added and the emulsion was cooled. 50 grams of this emulsion was heated to 40 C. and /3 of a gram of orcinol was added. One milligram of 2,3'-diethyl-4'-methyl-oxathiazole-carbocyanine dissolved in a little methyl alcohol was added to sensitize the emulsion. The emulsion was coated on a glass plate and found to gel in 2-3 minutes at room temperature and more rapidly when chilled. The plate was exposed wet and processed. It was found to develop and fix readily.

A plate coated as before was washed by placing in cold water for 30 minutes and was then dried. On exposing and processing, the plate was found to develop and fix readily giving a clear, clean result. The emulsion was also coated on paper. It was dried by subjecting to a current of warm air. Upon exposing and processing, it was found to give a clear, clean result.

Example III 15 grams of a 10% solution of polyvinyl alcohol was diluted with 80 cc. of water. 2 grams of urea and 10 cc. of 28% ammonia water were added and the solution was heated to 80 F. Into this the following solutions were run simultaneously with stirring over a period of three minutes. Solution A: 10 grams of silver nitrate made up to 20 cc. with water, Solution B: 8 grams of potassium bromide and 0.25 gram of potassium iodide made up to 21 cc. with water. grams of 10% polyvinyl alcohol solution was then added followed by the addition of 10 cc.

of glacial acetic acid. The emulsion was then heated to 145 F. and 20 cc. of a 6% solution of gallic in ethanol was added. The emulsion was boiled for 16 hours. It set to a firm gel which was cut into strips and thoroughly washed with water. The washed emulsion was melted by heating to F. and 10 cc. of a 6% solution of gallic acid was added thereto. The emulsion was coated on glass plates. It set to a gel and dried rapidly. These plates were exposed and processed. They were found to be of medium speed and to give a clear, clean result.

Example IV 15 grams of a 10% solution of polyvinyl alcohol was diluted with 80 cc. of water. 2 grams of urea and 10 cc. of 28% ammonia water were added and the solution was heated to 80 F. Into this the following solutions were run simultaneously with stirring over a. period of three minutes, Solution A: 10 grams of silver nitrate made up to 20 cc. with water, Solution B: 8 grams of potassium bromide and 0.25 gram of potassium iodide made up to 21 cc. with water. 130 grams of 10% polyvinyl alcohol solution was then added followed by the addition of 10 cc. of glacial acetic acid. The emulsion was then heated to F. and 20 cc. of a 12% solution of orcinol in ethanol was added. The emulsion was boiled for 16 hours. It set to a firm gel which was cut into strips and thoroughly washed with water. The washed emulsion was melted by heating to 140 F. and 10 cc. of a 12% solution of orcinol was added thereto. The emulsion was coated on glass plates. It set to a. gel

and dried rapidly. The plates were exposed and processed. They were found to be of medium speed and to give a clear, clean result.

I claim:

1. A light-sensitive photographic emulsion comprising a silver halide dispersed in a thermal-reversible gel composed of polyvinyl alcohol mixed with a gelling amount of substituted hydroxy aromatic compound selected from the group consisting of orcinol, gallic acid, 2-4 dihydroxy benzoic acid, 4-chlor resorcinol, and the substituted monohydric naphthols.

2. A light-sensitive photographic emulsion comprising a silver halide dispersed in a thermal-reversible gel composed of polyvinyl alcohol mixed with a gelling amount of orcinol.

3. A light-sensitive photographic emulsion comprising a silver halide dispersed in a thermalreversible gel composed of polyvinyl alcohol mixed with a gelling amount of gallic acid.

4. A light-sensitive photographic emulsion comprising a silver halide dispersed in a thermal-reversible gel composed of polyvinyl alcohol mixed with a gelling amount of a substituted hydroxy aromatic compound selected from the group consisting of orcinol, gallic acid, 2-4 dihydroxy benzoic acid, and 4-chlor resorcinol.

5. A light-sensitive photographic emulsion comprising a silver halide dispersed in a thermal-reversible gel composed of polyvinyl alcohol mixed with a gelling amount of a substituted monohydric naphthol.

6. A light-sensitive photographic emulsion comprising a silver halide dispersed in a thermal-reversible gel composed of polyvinyl alcohol mixed with a gelling amount of S-amino-lnaphthol hydrochloride.

7. A method of preparing a light-sensitive photographic emulsion which comprises preparing a silver halide in an aqueous solution of polyvinyl alcohol and subsequently adding thereto at a temperature of 40-90 C. a sufficient amount of substituted hydroxy aromatic compound selected from the group consisting of orcinol, gallic acid, 2-4 dihydroxy benzoic acid, 4-chlor resorcinol, and the substituted monohydric naphthols to form a rigid gel at a lowered temperature.

8. A method of preparing light-sensitive photographic emulsion which comprises preparing a silver halide in an aqueous solution of polyvinyl alcohol and subsequently adding thereto at a temperature of 40-90 C. a sufficient amount of orcinol to form a rigid gel at a lowered temperature.

9. A method of preparing light-sensitive photographic emulsion which comprises preparing a silver halide in an aqueous solution of polyvinyl alcohol and subsequently adding thereto at a temperature of 40-90 C. a sufficient amount of gallic acid to form a rigid gel at a lowered temperature.

WESLEY G. LOWE. 

